Transition from a single-ion to a collective diffusion mechanism in alkali borate glasses

Abstract Two distinct regions of the dc conductivity and its temperature dependence as a function of the mol fraction of alkali oxides, X , are observed in Na- and Li-borate glasses. At low alkali contents the dc conductivity σ dc increases only moderately with X . However, at higher alkali contents, log σ dc increases linearly and the activation enthalpy Δ H of σ dc  ×  T decreases linearly with log X , i.e. the dc conductivity reveals an effective power-law behavior. The transition between low alkali and high alkali behavior takes place at X  ≈ 0.08 for Na-borate and at X  ≈ 0.09 for Li-borate glasses. This behavior suggests that the diffusion mechanism changes at these alkali contents. The results are discussed in terms of ion separations and the transition from a single-ion jump to a collective diffusion mechanism. The vanishing of the mixed-alkali peak in Na–Rb borate and alumino-germanate glasses at sodium contents similar to that observed for the change in slope of σ dc ( X ) in this work suggests that both phenomena share the same origin.